The invention relates to the identification of the molecular basis of supravalvular aortic stenosis (SVAS) and Williams syndrome. More specifically, the invention has identified that elastin causes or is involved in the pathogenesis of SVAS and Williams syndrome. Molecular variants of the elastin gene contribute to SVAS and Williams syndrome. The analysis of the elastin gene will provide an early diagnosis of subjects with SVAS and Williams syndrome. The diagnostic method comprises analyzing the DNA sequence of the elastin gene of an individual to be tested and comparing it with the DNA sequence of the native, non-variant elastin gene. In a second embodiment, the elastin gene of an individual to be tested is screened for mutations associated with SVAS or Williams syndrome. Presymptomatic diagnosis of SVAS and Williams syndrome will enable practitioners to prevent vascular obstruction using existing medical therapies like beta adrenergic blocking agents.